1. Field of the Invention
The present invention relates to a concentration detector.
2. Description of the Related Art
Japanese Unexamined Patent Publication (Kokai) No. 2000-202285 discloses a device (hereinafter referred to as a “NOx detector”) for detecting the concentration of nitrogen oxides (NOx) in the exhaust gas emitted from the combustion chambers of an internal combustion engine. This NOx detector includes a cell for discharging oxygen from the exhaust gas that has flowed therein (this cell in Japanese Unexamined Patent Publication (Kokai) No. 2000-202285 is designated at 110 and is called a “pump cell” and is, hence, referred to hereinafter as a “pump cell”), a cell for producing an output value corresponding to the oxygen concentration in the exhaust gas after oxygen has been discharged by the pump cell (this cell in Japanese Unexamined Patent Publication (Kokai) No. 2000-202285 is designated 120 and is called a “monitor cell” and is, hence, referred to hereinafter as a “monitor cell”), and a cell which forms oxygen by decomposing NOx in the exhaust gas after oxygen is discharged and produces an output value corresponding to the oxygen concentration in the exhaust gas after oxygen has been formed (this cell in Japanese Unexamined Patent Publication (Kokai) No. 2000-202285 is designated 130 and is called a “sensor cell” and is, hence, referred to hereinafter as a “sensor cell”).
In the NOx detector disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2000-202285, the pump cell discharges oxygen from the exhaust gas such that the oxygen concentration in the exhaust gas drops to a predetermined value. On the other hand, the monitor cell produces an output value (hereinafter referred to as a “monitor cell output value”) corresponding to the oxygen concentration in the exhaust gas after oxygen is discharged by the pump cell. The sensor cell forms oxygen by decomposing NOx in the exhaust gas after oxygen is discharged by the pump cell and, then, produces an output value (hereinafter referred to as a “pump cell output value”) corresponding to the oxygen concentration in the exhaust gas. Therefore, simply speaking, a value obtained by subtracting the monitor cell output value from the sensor cell output value will correspond to the NOx concentration in the exhaust gas. Therefore, if this value is suitably converted, then, the NOx concentration can be obtained.
As described in Japanese Unexamined Patent Publication (Kokai) No. 2000-202285 (see paragraph 0125), however, the monitor cell and the sensor cell are different in their sizes and in their constituent materials. Therefore, the output characteristics of the monitor cell are not in perfect agreement with the output characteristics of the sensor cell. Namely, a value obtained by simply subtracting the monitor cell output value from the sensor cell output value does not correspond to the NOx concentration in the exhaust gas. According to Japanese Unexamined Patent Publication (Kokai) No. 2000-202285, therefore, a coefficient (hereinafter referred to as “correction coefficient”) is found for correcting the difference between the output characteristics of the monitor cell and the output characteristics of the sensor cell, the monitor cell output value is multiplied by this coefficient, the obtained value is subtracted from the sensor cell output value, and the NOx concentration in the exhaust gas is calculated based on the thus obtained value.
In the NOx detector of the above-mentioned type, the operation is necessary for bringing the output characteristics of the monitor cell into agreement with the output characteristics of the sensor cell in order to correctly calculate the NOx concentration. According to Japanese Unexamined Patent Publication (Kokai) No. 2000-202285, this operation is conducted by multiplying the monitor cell output value by the above correction coefficient that is found in advance.
Here, the output characteristics of the monitor cell and of the sensor cell vary as the NOx detector is used. Namely, as the NOx detector is continuously used, the output characteristics of the monitor cell and of the sensor cell vary due to aging of the monitor cell and the sensor cell. To correctly calculate the NOx concentration, therefore, the correction coefficient must be varied while the NOx detector is being used. If the correction coefficient that is found in advance is continuously used, then, it becomes difficult to correctly calculate the NOx concentration. This generally applies even when a particular component is calculated (detected) in the gas.
It is therefore an object of the present invention to correctly calculate the concentration of a particular component in the gas.